In an auxiliary unit drive system that drives, by the power of an engine for a vehicle, an auxiliary unit such as an alternator, a belt is stretched over a pulley connected to a drive shaft of the auxiliary unit such as the alternator and a pulley connected to a crankshaft of the engine, and torque of the engine is transmitted to the auxiliary unit via the belt. Typically, the rotating speed of the crankshaft of the engine changes due to a power stroke of the engine or the like, and the travelling speed of the belt also changes. For this reason, a slip may occur between the pulley connected to the drive shaft of the auxiliary unit and the belt, or the tension of the belt may change considerably. The slip of the belt or an excessive change in the tension of the belt is one of causes of the occurrence of an abnormal sound or lifetime depression of the belt.
In particular, since a power generating shaft of the alternator has a large moment of inertia, a slip of the belt or a change in the tension of the belt is likely to occur. There are problems in that the transmission of a change of rotation of the crankshaft to the power generating shaft decreases the durability of the alternator and adversely affects a power generating efficiency.
In the related art, various proposals for a pulley are made so as to absorb a change in the rotation of the crankshaft. For example, a pulley structure disclosed in Patent Document 1 contains a first rotatable body over which a belt is stretched; a second rotatable body that is provided inward of the first rotatable body and can rotate relative to the first rotatable body; and a coil spring that is disposed between the two rotatable bodies. Outer circumferential surfaces (or inner circumferential surfaces) of opposite end portions of the coil spring are in press contact with the first and second rotatable bodies, respectively due to a self restoring force in a diameter increasing direction (or diameter decreasing direction).
When a change in the rotation of the crankshaft is transmitted to the first rotatable body via the belt and the two rotatable bodies rotate relative to each other, torque is transmitted between the two rotatable bodies via the coil spring interposed therebetween, the coil spring is twisted in a circumferential direction, so that the change of rotation is absorbed. For this reason, it is possible to prevent the occurrence of a slip of the belt or a change in tension.